The potential demand for ubiquitous wireless communications combined with restricted availability of the radio frequency spectrum has motivated intense research into bandwidth efficient multiple access schemes. A recent reference entitled "Spread Spectrum for Commercial Communications", by Schilling et al, as published in IEEE Communications Magazine, Vol. 29, No. 4, April 1991 discusses one avenue of approach, namely, Spread Spectrum Code Division Multiple Access (SS-CDMA) techniques, to effect multiple access communication.
Conventional CDMA techniques take advantage of available bandwidth on the transmission medium, such as a fiber optic cable or the radio spectrum, by generating a set of pulses in the time domain which have appropriate correlation properties over predetermined time periods. Typically, the correlation property is such that a particular receiver tuned to a given transmitter code produces a detectable signal whenever the given transmitter code is presented to the receiver during each time period, whereas the output of the receiver is near zero for any other transmitter code presented to the receiver. A CDMA system operating on this time domain correlation property and utilizing a set of codes designated the optimal orthogonal codes was disclosed in U.S. Pat. No. 4,779,266.
In the article entitled "Coherent Ultrashort Light Pulse Code-Division Multiple Access Communication Systems", appearing in the Journal of Lightwave Technology, by J. A. Salehi, A. M. Weiner, and J. P. Heritage, March, 1990, Vol. 8, No. 3, a technique for encoding a sequence of ultrashort pulses for transmission over an optical channel is disclosed. The encoding is effected by modulating the phase characteristic of the Fourier transform of a stream of ultrashort light pulses corresponding to a sequence of data symbols. Each transmitter is assigned a unique code to modulate its corresponding stream, and a receiver tuned to that unique code can detect the data symbols impressed on the short pulses by the associated transmitter. As disclosed, encoding is accomplished, at optical frequencies, by an arrangement of grating elements and a multielement phase modulator.
There is no teaching or suggestion in the art of any technique to generate a transmitter pulse which is power limited and which must match to a channel having bandwidth restrictions, additive noise, and a frequency transfer response characterized by loss and distortion. An example of such a channel is one which supports transmission only on disconnected frequency bands.